Can I Limit #of People in 1 Bdrm Apt?

[Mr Good]

Hi. I have a duplex in a decent area, and "knock on wood" I have had full vacancy and very few problems in about 18 months.

I have a tenant moving in Sept-01, single lady hairdresser. She says it's just her, but when she came to see it she brought her boyfriend (security I guess). He seems nice enough. The rental is large but has 1 bedroom.

My only concern is that this person will move her boyfriend in, then her mother, and niece, etc.

Question: legally (in WA state) how many people can I restrict to living in a 1 bedroom? Thanks for any advice.